Many designs of radiotherapy or radiation imaging apparatus employ a radiation source (such as a linear accelerator) mounted on a gantry arm that is rotatable around a patient support on which a patient can be placed for treatment, usually lying face upwards or supine. The usual geometry for this comprises a generally cylindrical, rotatable drum with a diameter of about 2 meters oriented in a vertical plane, i.e. with its axis of symmetry and rotation in a horizontal plane, with the gantry arm mounted onto the drum so that it projects outwardly from a circular face of the drum in cantilever fashion, offset from the axis of the drum. The radiation source is mounted at the end of the gantry arm distant from the drum, oriented so that the beam of radiation that it produces is directed towards the axis of rotation of the drum. The point at which the centre of the beam meets the axis is known as the “isocentre”. In other designs there is no gantry arm, instead the radiation source is mounted to the cylindrical drum directly, usually towards its outer circumference. In all of these systems, as the drum rotates, the beam arrives at the isocentre from all angular directions within a vertical plane. This is an important aspect of radiotherapy treatment, as it allows a sufficient dose to be delivered to a target volume while minimising the dose delivered to surrounding healthy tissue, and it is important in rotary imaging, as it allows a very accurate image of a region of interest to be compiled from multiple images taken at many different angles.
Usually, the rotating drum is supported on a static support structure comprising four main wheels beneath the drum, arranged in two angularly-offset pairs, one pair at a front edge of the drum and one pair at a rear edge; alternatively there may be a pair of rollers extending between the front and rear edges of the drum. The drum and (where present) the gantry arm are usually very substantial items in order to support the weight of the radiation source mounted in the arm and at the end thereof—in modern radiotherapy systems the drum and gantry weigh between about 4 and 7 tonnes, and this weight is increased when the drum also supports another cantilever arm supporting further device, for example an imaging device (such as an EPID—Electronic Portal Imaging Device), opposite the radiation source for acquiring images of the patient from the attenuated radiation after it has passed through the patient's body. All forms of rotary drums, from a drum which has no gantry arm and is substantially cylindrical, to a drum which has one or more gantry arms supporting one or more devices are collectively referred to herein as “generally cylindrical rotary elements”.
The rotation may be relatively constant in speed and/or direction, or it may “step” from one angular position to another, and/or it may change direction of rotation. The rotation speed is normally relatively slow, partly because the rotating parts are so massive and causing them to rotate and to stop or to change direction requires significant energy, and partly for reasons of health and safety. Health and safety also requires that the parts which are rotating in close vicinity to the patient can be brought quickly to a halt over a short angular distance, such as in an emergency; in order to stop a rotating body, a braking torque must be applied, and this torque must also be resisted by the supporting structure. There is a need to reduce the time taken for radiation imaging and treatment, so as to minimise the time each patient needs to spend inside the apparatus, and to allow more patients to be treated each day. One way of reducing treatment time would be to increase the rotational speed of the drum; however, stopping the rotation of a relatively massive device over a short angular distance exerts substantial reaction forces on the apparatus and the structure to which it is mounted; increasing the speed of rotation, and/or reducing the time for changes of rotational direction increases these forces to the extent that there is the possibility of slippage between the drum and the wheels, a need for multiple drive wheels to accelerate and decelerate the drum, and there are significant lateral loads to be borne by the base supporting structure for the drum. Dealing with these concerns necessitates extensive and expensive additions or modifications to be made to the apparatus, or in extreme circumstances it may not be practicable to address some or all of them and so the drum rotation speed would have to be reduced.